<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div{
            width: 100px;
            height: 100px;
            border: 1px solid red;
        }
        #d1{
            /*元素脱离文档流 让出所占位置 后面元素顶上来*/
            position: absolute;/*绝对定位*/
            /*绝对定位坐标对于窗口*/
            left: 0;
            top: 0;

        }
    </style>
</head>
<body>
<div id="d1">div1</div>
<div ud="d2">div2</div>
</body>
</html>